Refining acid anhydrides



- adequately removed.

Patented Jan. 26, 1943 UNITED STATES, PATENT OFFICE REFINING ACIDANHYDRIDES William Charles Cooper, signor, by mesne assl York Jr.,Jenkintown, Pa, as-

gnments, to Allied Chemical & Dye Corporation, a corporation of New NoDrawing. Application April 22, 1940, Serial No. 330,954

10 Claims. (01. 2so e4s) various impurities. In the past it has beenproposed to purify such phthalic anhydride physically bydistillation orby sublimation, inwhich case some color-forming impurities remain, orchemically :by a heat treatment alone or with condensing agents such assulfuric acid, oleum,

and other acidic substances. If the heat-treat ment without a condensingagent is employed, a long heating period is required and even thenodorous and color-forming impurities are not The product moreover tendsto become colored or to darken with age. The chemical methods apparentlyowe their effectiveness to an ability to destroy the impuritiesoriginally present in the phthalicanhydride,

. such as volatile quinonic impurities; at the same time, however, theyintroduce other impurities. By distilling or subliming the heat-treatedphthalic anhydride, the impurities possessing or producing color -may bepartially eliminated. Other impurities introduced by acid-treatment arenot eliminated by such distillation or sublimation. Moreover, theseimpurities have a deleterious effect upon the color stability of theanhydride products, and alter the chemical characteristics oftheproducts so that when employed in resin manufacture, they yieldirregular and annoyingly unpredictable results.

Maleic anhydride produced by catalytic vapor phase air oxidation ofbenzene and other organic compounds also is contaminated withimpurities. It has been proposed to subject the crude maleic anhydrideto a heat-treatment for a number of hours to convert volatile impuritiesto less volatileproducts and then to distill off relatively pureanhydride product from the less volatile products. Although thisprocedure is not subject to the disadvantage of introducing chemicallyactive or activating impurities into the final prodnot, the method isdisadvantageous in that the product has only fair color stability.

In accordance with the present invention I have found that maleicanhydride and phthalic anhydride may be purified by treatment with anacidic condensing agent and subsequent treatment with an alkalinereagent to produce products of superior quality.

By my process maleic and phthalic anhydrides may be produced not only insubstantially colortion of the anhydrides in the manufacture of.

less condition but at the same time free from impurities affecting thenature or rate of reac- The products of the present invention are alsounusually satisfactory for manufacture of esters in that in addition tofreedom from reactive and color-forming, and possibly catalytic,impurities they are relatively free from disagreeable odor, and theester products producible therefrom are likewise of good odor.

While it was to be expected, in view of the large proportion of phthalicor maleic anhydrlde and. minute proportions of acidic impuritiespresent, that small quantities of alkaline reagents would react with-theanhydride rather than withthe acidic impurities, I have found that byproper control of the temperature during'the alkaline treatment thealkaline reagent may be caused to react preferentially with the acidicimpurities so as to fix themin a relatively non-volatile form. By asubsequent distillation of the phthalic or maleic anhydride the refinedanhydride may be obtained as distillate while the various impuritiescontributing undesirable properties to the product are retained asdistillation residue.

For the acid-treatment step of my purification process, I have foundacidic condensing agents, such as 66 B. sulfuric acid, oleum, stannicchloride, and sulfur trioxide, to be suitable.

The amount of acidic reagent required is nor- 300 C. to be particularlysuitable for the acidtreatment in the purification of phthalicanhydride, whereas temperatures between and C. are quite satisfactoryfor the acid-treatment of maleic anhydride.

For the alkaline treatment I have-found weakly alkaline reagents, e. g.,soda ash, sodium bicarbonate, barium hydroxide, or borax, to beparticularly satisfactory and these are my preferred alkaline reagents.Other alkaline-reacting materials, such vas caustic alkalies, quicklime,zinc oxide, and alumina may be usedbut to less advantage than thefirst-mentioned materials. Caustic alkalies are less desirable than thepreferred alkaline reagents since the caustic alkalies tend to reactwith the organic anhydride and thus reduce the yield of desired product.

A mild alkaline treatment, that is, a treatmentv with sufficiently lowconcentration of reagent and temperature of reaction to avoid charring"or In the purification of phthalic anhydride I have found temperaturesbetween 150 and 250 C. to be quite satisfactory for the alkaline treatment. Temperatures substantially above 250 C. should not be employedsince at such temperatures the alkaline reagent is reacted upon by thephthalic anhydride. Under such circumstances charringof the phathalicanhydride occurs with the formation of apparently quinone-llke bodieswhich are detrimental to the color of the finished product and loss ofphthalic anhydride is occasioned by the interaction of phthalicanhydride and the alkaline reagent. In the purification of maleicanhydride temperatures between 100 and 150 C. may be used for thealkaline treatment as well as for the acid-treatment. It is desirable toavoid temperatures above 150? C. for both the acidic and alkalinetreatments of maleic anhydride since at higher temperaturesl have foundthat the anhydride tends to form condensation, polymerization, or othertypes of reaction products which adversely affect th yield and qualityof the anhydride product. a

The anhydride being purified may be sub- Jected to a distillation notonly after the alkaline treatment butalso between the acid-treatment andthe alkaline treatment. This step is advantageous in that it provides ahigher degree of purification and reduces the quantityof alkalinereagent required. Moreover, the distillation residue obtained in each ofthe distillations is more easily handled than that obtained when onlyone distillation is employed. The intermediate distillation, of course,requires additional heating and distillation capacity and it frequentlyis preferable to accept the slightlylower purity obtain able with onedistillation than to expend the heat and still time required for twodistillations.

The time required for the purification treatment usually variesinversely with the tempera.- ture employed. For the higher treatmenttemperatures mentioned above treatment periods of I about V4, to 1 hourare satisfactory for the acidtreatments. At the lower temperatures thetreatment periods may be extended for an additlonal hour or two. Thealkaline treatments may be cdnducted for periods varying from A hour upto 2- or 3 hours. The treatmentperiods given are representativ foroperations with agitating equipment of ordinary efiiciency. Shorterperiods may be employed with highly effective agitators. Longer periodsdo no harm.

The following examples illustrate the purification process of theinvention.

Example 1 A phthalic acid product, obtained by catalytic vapor phase airoxidation of naphthalene and condensation from the product gases bydirect contact of the gases with a phthalic acid slurry,-

was thermally dehydrated and distilled to provide a semi-refinedphthalic anhydride. The

semi-refined product was heated to a temperature of about 280 C. atordinary atmospheric pressure (760 mm.-of mercury). To the hot moltenphthalic anhydride distillate of its weight of 66 B. (98%) sulfuric acidwas added during agitation of the mass. After additipn of the sulfuricacid, agitation at a temperature of about 280 C. was continued for about1 hour at atmospheric pressure. The temperature of the batch was thenallowed to fall to about 200' C. and while agitation was continued. byweight of soda ash was added to the molten anhydride. Agitation wascontinued for an additional hour at approximately the same temperatureand pressure. The-phthalic anhydride was then subjected to fractionaldistillation at an absolute pressure of about 75 mm. of mercury.

The first fraction and the residue consist of V phthalic anhydride andmore vobatile and less volatile impurities and may be separatelydistilled or otherwise processed to recover their phthalic anhydridecontents. For instance, it is feasible to distill each of these productsto obtain phthalic anhydride containing a substantial quantity ofimpurities and to add such phthalic anhydride to the next batch ofsemirefi'ned anhydride or crude anhydride to be processed.

The refined phthalic anhydride product in molten condition has a colorof about 0 by the Platinum-Cobalt (Hazen) Color standards; it

may be reacted with glycerin to form light-colored resins.

Example 2 A light-yellow maleic anhydride produced by dehydration ofmaleic acid and having a melting point of approximately 52 C., ismelted, freed from contained moisture and then heated to about C. atordinary atmospheric pressure. An amount of 66 B. sulfuric acid equal toof the weight of maleic anhydride is then added and the batch isagitated for about 1 hour.

Soda ash, amounting to about by weight of the batch, is then added andagitation continued By fractional distillation at for another hour.ordinary atmospheric pressure, a water-white distillate of a meltingpoint of 52.6 to 53.0 C. is obtained.

It will be understood that the term distillation as used in the appendedclaims includes vaporization from solid as well as from liquid phase. 1I claim:

l. The method of refining a dicarboxylic acid anhydride, which comprisessubjecting the anhydride in molten phase to an acid-treatment,subjecting the acid-treated anhydride in molten phase to a mild alkalinetreatment, and separating the anhydride from less volatile reactionproducts by vaporization.

2..The method of refining a crude dicarboxylic acid anhydride, whichcomprises distilling the crude anhydride to provide a semi-refinedanhydride product, subjecting the aemi-rea I fined product in moltenphase to an acid-treatment, distilling oil. dicarboxylic anhydride fromless volatile reaction products of the acid-treatment, subjecting theacid-treated anhydride distillate in molten phase to a mild alkalinetreatment, and subsequently separating the anhydride from less volatilereaction products by distillation.

3. The method of refining phthalic anhydride, which comprises subjectingthe anhydride in molten phase to an acid-treatment at a temperaturebetween 250 and 300 0., subsequently subjecting the acid-treatedanhydride to treatment with a weakly alkaline reagent at a temperaturebetween 150 and 250 0., and. thereafter separating the refined phthalicanhydride from less volatile reaction products by vaporization.

4. The method of refining phthalic anhydride, which comprises treatingmolten phthalic anhydride with a small proportion of concentratedsulfuric acid at a temperature between 250 and 300 0., cooling theanhydrideto a temperature between l50'and 250 0., adding a smallproportion of an alkali-metal carbonate to the thus cooled moltenanhydride, and thereafter distilling phthalic anhydride from lessvolatile products produced by the sulfuric acid and alkalimetalcarbonate treatments.

5. The method of refining phthalic anhydride, which comprises treatingmolten phthalic anhydride at a temperature between 250 and 300 C. with asmall proportion of concentrated sulfuric acid, distilling off phthalicanhydride from less volatile reaction products of the acid-treatment,

- treating the phthalicanhydride distillate at a temperature between 150and 250 0.-wi th a small proportion of alkali-metal carbonate,andphthalic anhydride product, treating the semireflned product inmolten phase at a temperature between 250 and 300 C. with between 0.1%and 5% by weight of concentrated sulfuric acid, distilling oil phthalicanhydride from less volatile reaction products of the acid-treatment,

treating the phthalic anhydride distillate at a temperature between 150and 250 0. with between 0.1% and 5% by weight of an alkali-metalcarbonate, and distilling elf phthalic anhydride from less volatileproducts of the carbonate treatment.

8. The method of refining maleic anhydride, which comprises subjectingthe anhydride in molten phase to an acid-treatment at a temperaturebetween 100 0. and 150 0., subjecting the acid-treated anhydride inmolten phase to treatment with a weakly alkaline reagent at atemperature between 100' C. and 150 C., and

separating the maleic anhydride from less volatile reaction products byvaporization. I

9. The method of refining maleic anhydride, which comprises treatingmolten maleic anhydride at a temperature between 100 0. and 150' 0. witha small proportion of concentrated sulfuric acid, adding a smallproportion of alkalimetal carbonateto the molten maleic anhydride whilethe anhydride is at a temperature between 100 0. and 150 0., andthereafter distilling of! maleic anhydride from less volatile productsof the sulfuric acid and alkali-metal carbonate treatments.

10. The method of refining maleic anhydride,

' which comprises treating molten maleic anhyacid at a temperaturebetween 250 and 300 0.. f

cooling the anhydride to a temperature between 150 and 250 0., addingbetween 0.1% and 5% by'weight of an alkali-metal carbonate to the thuscooled molten anhydride, and thereafter distilling phthalic anhydridefrom less volatile dride at a temperature between 100 0. and 150 0. withbetween 0.1% and 5% by weight of con- 2 centrated sulfuric acid, addingbetween 0.1%

i and 5% by weight of an alkali-metal carbonate products produced by thesulfuric acid and al- .kali-metal carbonate treatments. e V 7.'Themethod of refin ng crude phthalic anhydride, which comprises subjectingthe crude anhydride to distillation to form a semi-refined to the moltenmaleic anhydride while the anhydride is at a. temperature between 0. and0., and thereafter distilling oi! maleic an-' hydride. from lessvolatile products of the sulfuric acid and alkali-metal carbonate treat-'ments.

CHARLES COOPER, JR.

